We have previously reported recurrent clonal chromosomal aberrations in synovia, osteophytes and articular cartilage from patients with osteoarthritis (OA). In particular, gain of chromosomes 5 and 7 was found to be strongly associated with OA. In order to exclude the possibility of in vitro artefacts, we studied three to four parallel, independent cultures from ten samples of synovia and three samples of osteophytes from ten women with primary OA. In all, 40 cultures were cytogenetically analysed, 39 of which had clonal chromosomal aberrations. The most common aberrations were +7 and +5 which were found in 38 and 12 cultures, respectively. There were striking karyotype similarities among the parallel cultures from each case. Out of a total of 83 clones, only 11 were unique for one culture, 7 from synovia and 4 from osteophytes. The genetic homogeneity among different cultures from the same patients excludes the possibility of in vitro artefacts and indicates a widespread distribution of the cytogenetically aberrant clones in vivo.